John Mueller of Google explained on Twitter after being questions about the guidelines change, "Invalid HTML causes more problems now than years ago: structured data & mobile come to mind," he said. He added "but for the most part, we just wanted to rephrase it in a .. correct way :)."

This came up recently with the change of the guidelines, with regards to change made in the webmaster guidelines. We mentioned use valid HTML. The question here is Is the W3C Validation (Broken HTML) ranking factor or should we care about it?

It is not directly a ranking factor. It is not the sense that if your site is not using valid HTML we will remove it from he index. Because I think we will have a pretty empty search results.

But there are a few aspects there that do come into play. On the one hand, a site with really broken HTML, something that we see really rarely, then it is really hard for us to crawl it and index the content because we can’t pick it up.

The other two aspects which are kind more in regards to structured data. Sometimes it is really hard to pick up the structured data when the HTML is broken completely. So you can’t easily use a validator for the structured data.

The other thing is in regards to mobile devices and cross browser support is if you have broken HTML then that sometimes really hard to render on newer devices.

Here is that video:

Again, nothing new but he had to explain the wording change in the guidelines.